In a wireless communication system, a base station may provide one or more coverage areas, such as sectors, in which the base station may serve user equipment devices (UEs), such as cell phones, wirelessly-equipped personal computers or tablets, tracking devices, embedded wireless communication modules, or other devices equipped with wireless communication functionality. In general, each coverage area may operate on one or more carriers each defining a respective bandwidth of coverage, and each coverage area may define an air interface providing a downlink for carrying communications from the base station to UEs and an uplink for carrying communications from UEs to the base station. The downlink and uplink may operate on separate carriers or may be time division multiplexed over the same carrier(s). Further, the air interface may define various channels for carrying communications between the base station and UEs. For instance, the air interface may define one or more downlink traffic channels and downlink control channels, and one or more uplink traffic channels and uplink control channels.
In accordance with the Long Terminal Evolution (LTE) standard of the Universal Mobile Telecommunications System (UMTS), a base station (e.g., an eNodeB) may provide multiple sectors. In each sector, the base station may provide service on one or more carriers spanning 3 MHz, 5 MHz, 10 MHz, 15 MHz, or 20 MHz. Each carrier in a particular sector defines a “cell”. For example, if a base station provides three sectors and provides service on two carriers per sector, the base station provides six cells. In some examples, there could be multiple cells at the same physical location, each provided by the same base station, and each being on a different carrier.
In an LTE system, each eNodeB has a global eNodeB ID and each sector of an eNodeB has a sector ID. Further, since each carrier in a particular sector defines a cell, each cell of a sector has a cell ID. Thus, at the system level, each combination of global eNodeB ID and cell ID defines a globally unique identifier for a cell. This globally unique identifier is referred to as an Evolved Universal Terrestrial Radio Access Network (E-UTRAN) cell global identifier (ECGI). At the physical level, each cell provided by an eNodeB also has a physical cell identifier (PCI) that is identifiable by a UE. While an ECGI is globally unique within a public land network, there are only 504 possible PCIs. Thus, PCIs are likely to be repeated many times throughout a public land mobile network. In practice, each eNodeB may store a mapping between the PCI and cell ID of each of its cells.
Further, an LTE system may support handover of a UE from one cell to another. For instance, when a first eNodeB is serving a UE and the UE detects sufficiently strong coverage from a cell of a second eNodeB, the UE may send a report to the first eNodeB indicating the detected PCI of the cell of the second eNodeB. The first eNodeB may in turn determine if applicable handover thresholds are met. If so, the first eNodeB may engage in handover signaling via an interface with the other eNodeB to orchestrate handover of the UE to the reported cell.
In LTE, the handover signaling may occur over an interface used to interconnect eNodeBs, such as an X2 interface. When an X2 interface is provisioned between two eNodeBs, each eNodeB shares with the other neighbor eNodeB (i) its eNodeB global ID and (ii) each of its cell IDs and corresponding PCIs. Thus, an eNodeB may have a record of a neighbor eNodeB's cell IDs and, for each cell, the corresponding PCI. In addition, eNodeBs may use the X2 interface to report their respective load to their neighboring eNodeBs on a per-cell basis.
Furthermore, a revision of LTE known as LTE-Advanced now permits an eNodeB to serve a UE with “carrier aggregation,” by which the eNodeB schedules bearer communication with the UE on multiple carriers at a time. With carrier aggregation, multiple carriers from either contiguous frequency bands or non-contiguous frequency bands can be aggregated to increase the bandwidth available to the UE. Currently, the maximum bandwidth for a data transaction between an eNodeB and a UE using a single carrier is 20 MHz. Using carrier aggregation, an eNodeB may increase the maximum bandwidth to up to 100 MHz by aggregating up to five carriers. Each aggregated carrier is referred to as a component carrier. Further, when multiple carriers are aggregated, one of the component carriers may be defined as a primary cell (“PCell”) and the remaining component carriers may be defined as secondary cells (“SCells”).
Depending on the desired implementation, an eNodeB may be carrier aggregation capable or not. If an eNodeB is carrier aggregation capable, the eNodeB may have certain policies regarding which of its carriers, per sector, can be combined together to provide carrier aggregation. By way of example, an eNodeB may have a carrier-aggregation policy that indicates which carrier(s) can be used as a PCell in combination with which one or more carriers being SCell(s). For instance, in one scenario, a carrier-aggregation policy may indicate that a first carrier and a second carrier may be used as PCells, but a third carrier may not be used as a PCell. Further, the carrier-aggregation policy may indicate that (i) the first carrier can be a PCell with the second carrier as an SCell, but not with the third carrier as an SCell, but (ii) the second carrier can be a PCell with both the first carrier and the third carrier as SCells. Thus, if the first carrier is used as a PCell, the eNodeB may aggregate two carriers, but if the second carrier is used as a PCell, the eNodeB may aggregate three carriers.
In practice, an eNodeB may implement a carrier-aggregation policy for a number of reasons. As one example, certain pairs of carriers may be undesirable to combine because concurrent transmission on the two carriers could give rise to intermodulation distortion. For instance, concurrent transmission on two particular carriers may combine to produce an undesirable radio frequency (RF) byproduct. If an eNodeB engages in carrier aggregation with a UE on the two carriers, the UE or eNodeB may receive the RF byproduct, thus interfering with the transmissions on the two carriers. As another example, an eNodeB may implement a carrier-aggregation policy that is dynamically modified over time based on the level of congestion in the control channel region of a particular cell. For instance, if all control signaling is configured to occur on PCells rather than SCells, and a control channel region of the particular cell is threshold highly congested, then it may be desirable to avoid using that cell as a PCell, but allow the cell to be used as an SCell.